This work investigates the effect of size and concentration of nanoparticles on the effective gas-liquid surface tension of aqueous solutions of bismuth telluride nanoparticles functionalized with thioglycolic acid. The gas-liquid surface tension is obtained by solving the Laplace-Young equation under experimentally measured boundary conditions and droplet parameters. The results demonstrate that the gas-liquid surface tension depends on concentration as well as nanoparticle size. Solutions containing 2.5 and 10.4 nm nanoparticle diameters have been tested. For both, a minimum surface tension exists within the range of tested mass concentrations. The largest reduction in the surface tension (>50% versus bulk liquid) occurred for the 2.5 nm nanoparticle nanofluid. Accumulation and assembly of the charged nanoparticles at the liquid-gas interface was assumed to be responsible for the surface tension of the nanofluids investigated in this work.